In LTE-Advanced systems, which are being investigated for 3GPP, carrier aggregation (CA: Carrier Aggregation) techniques that simultaneously use a plurality of frequency bands (component carrier (CC: Component Carrier)) are employed with an object of improving the frequency utilization efficiency and the peak throughput (refer to Non-Patent Document 1).
In inter-site carrier aggregation (Inter-Site CA), which represents a form of carrier aggregation, a mobile terminal device (UE: User Equipment) simultaneously connects to a plurality of base station devices (eNB: E-UTRAN NodeB) that are installed at different locations. Further, the mobile terminal device communicates with the base station devices using a different frequency band (CC) for each base station device.
A network configuration is proposed in which, for the communication between base station devices and a mobile terminal device, the C-Plane and the U-Plane are separated and each is transmitted via different base station devices (refer to Non-Patent Document 2).
A macro base station device of a macro cell (Macro Cell) refers to a base station device that is deployed over a wide area, mainly for securing coverage for mobile terminal devices. As an example, a carrier frequency band such as an 800 MHz band is utilized. A small cell base station device of a small cell (Small Cell) refers to a base station device with a narrow coverage that is installed on a spot-by-spot basis for expanding the capacity. As an example, a high-frequency band such as a 3.4 GHz band is utilized.
In carrier aggregation, a base station device that manages the RRC (Radio Resource Control) connections between mobile terminal devices and base station devices is referred to as a PCell (Primary Cell). A base station device that is not a PCell is referred to as an SCell (Secondary Cell). The C-Plane represents a protocol for performing connection management and mobility control of mobile terminal devices. The U-Plane represents a protocol that relates to the transmission and reception of user traffic and scheduling information.
A downlink (DL: Down Link) represents communication in the direction from a base station device to a mobile terminal device. An uplink (UL: Up Link) represents communication in the direction from a mobile terminal device to a base station device.